1. Field of the Invention
This invention relates to roofing compositions having a decorative metallic appearance. More particularly, the invention relates to roofing granules and to roofing shingles incorporating said granules possessing the characteristics of metallic brilliance, color and stability against discoloration/degradation.
2. Reported Developments
Roofing granules, both natural and artificially color-coated granules, are extensively used in roll roofing and asphalt shingle compositions. The roofing granules are typically embedded in the asphalt coating on the surface of an asphalt-impregnated felt base material, the granules thus forming a coating that provides a weather-resistant exterior roofing surface. In addition to its utility, the granule coating also provides an esthetic effect observable with respect to the coating composition, the appearance of the granules is of major marketing interest. For this reason a pigmented color coating is ordinarily applied to the base mineral granules to enhance their visual, decorative effect. The method of providing artificially colored roofing granules involves crushing and screening minerals and applying to the so-obtained granules a coating containing the desired pigment. The granules and the pigment are bonded together by using a soluble silicate binder solution which is then insolubilized by heat treatment alone or by chemical action to a water-insoluble state. The function of the soluble silicate is to first uniformly distribute and encapsulate the pigment within the coating matrix and then to bind the pigment and granules together when the silicate is insolubilized.
The soluble silicate binder is converted to an insoluble state by heat treatment as in the case of a silicate/clay coating matrix or by a combination of heat treatment and chemical action such as by the addition of an acidic material to the fired soluble silicate.
Exemplary references directed to the production of artificially colored granules and stabilizing the pigment therein are as follows.
U.S. Pat. No. Re. 20,295 describes artificially colored roofing granules produced by: depositing on granular mineral matter a metal salt followed by the addition of a soluble silicate which will react to form an insoluble metal glass; and subjecting the insoluble metal glass to heat in order to fuse the metal silicate on the granules.
U.S. Pat. No. 2,001,448 describes a method of producing artificially colored granules by the steps of: providing base particles, such as sand, gravel, rock, blast-furnace slags, or burned clay or shale and crushed brick; mixing the particles with a hydrous plastic clay in either wet or dry condition so that the clay coats the exterior surfaces of the particles; adding a suitable color pigment to coat the particles therewith; drying the particles; adding silicate of soda either per se or with a coloring pigment therein to react with the clay coating covering the particles; and heating the particles to a temperature of about 450xc2x0 to 1200xc2x0 F. to fuse together the coatings and the particles.
U.S. Pat. No. 2,695,851, describes artificially colored roofing granules and a method for producing them. The method comprises: forming on the base granules a coating of film-forming composition which contains a pigment, an alkali silicate solution, finely-divided hydrated alumina, and an insolubilizing agent comprising of clay, cryolite, feldspar, aluminum fluoride and sodium fluorosilicate; and heating the so-obtained granules to about 1000xc2x0 F.
The hydrated alumina used in combination with a light-colored, opaque pigment, such as titanium dioxide produces lighter and brighter granules than can be produced with titanium dioxide in the absence of hydrated alumina.
U.S. Pat. No. 2,732,311 is directed to a coating process for producing radiation-reflective roofing granules.
In the process the raw granules are mixed with a clay-like material, such as kaolin at a temperature below 100xc2x0 F., preferably at room temperature. A solution of sodium silicate is added to the mixture. Alternatively, the clay and sodium silicate can be pre-mixed then applied to the granules. The granules are then partially air-dried to develop a tacky film on the granules. When the proper viscous tacky film has been formed, aluminum flakes are then added to the mixture and are caused to adhere to the coated granules by further mixing. Instead of aluminum flakes, copper flakes, brass flakes, or metallic particles known as bronze powders and aluminum bronze may be used. The metal flake-coated granules are then fired at a temperature of about 900xc2x0 to 1000xc2x0 F.
U.S. Pat. No. 2,981,636 describes artificially colored roofing granules and a method for their preparation. The method of producing the colored roofing granules involves the steps of:
mixing the cleaned and heated granules with an aqueous solubon of sodium silicate; drying and dehydrating the mixture of granules and the aqueous solution; and adding an insolubilizing agent containing a desired pigment therein to the dehydrated granules.
The insolubilizing agent is a mixture of aluminum chloride and ammonium chloride, however, other insolubilizing agents may also be used, such as aluminum sulfate, magnesium chloride, zinc chloride, and hydrochloric acid.
It is apparent from the prior art that the firing temperature requirements, the highly alkaline coating matrix, and lack of coating transparency have made it difficult to prepare granules having a bright metallic luster. Past attempts have produced colored granules of poor weathering characteristics exhibiting rapid loss of color and metallic luster.
In the practice of the present invention, highly desirable properties are imparted to roofing granules in a manner providing enhanced metallic brilliance and color and excellent, long-term weathering characteristics.
In one aspect the present invention is directed to the manufacturing of artificially colored roofing granules comprising the steps of:
a) crushing and sizing a base aggregate to the desired size, typically to No. 11 grading;
b) preheating the sized/graded granules to about 175-190xc2x0 F.;
c) preparing a coating composition comprising: water, alkali silicate solution, and a silica-encapsulated metallic flake pigment in either powder or aqueous paste form, or a lamellar effect pigment such as a metal oxide-coated mica;
d) combining the preheated, sized/graded granules and the coating composition in xe2x80x9ccxe2x80x9d above by mixing to produce individually coated granules;
e) pre-drying the individually coated granules to reduce their moisture content to about 0.1%-0.5%;
f) kiln-firing the granules at about 400xc2x0 F. to form a transparent, partially insolubilized coating on the granules in which the metallic flake pigments are uniformly distributed;
g) cooling the granules to reduce their temperature to about 140-160xc2x0 F. by means of air flow and water spray application;
h) applying together with the water spray application a 28% aluminum chloride solution at a rate of about 12-16 pounds per ton (hereinafter sometimes termed as PPT) to the coated granules to allow chemical reaction to occur between the aluminum chloride and the alkali silicate to complete insolubilization; and
i) treating the coated granules with a mixture of process oil and an organosilicon compound to impart dust control, and to improve adhesion to a substrate, such as asphalt.
The base aggregate can be either untreated base aggregate or granules that have been colored by a conventional high-temperature silicate/clay coating process.
Optionally, the metallic flake pigment can be used in powdered form in conjunction with water. However, there is a significant safety advantage in using aluminum in aqueous paste from, as well as silica-encapsulated, to reduce the chance of dust ignition. With lamellar effect pigments consisting of doped metal oxide flakes or metal oxide-coated mica, however, there is no ignition hazard, so they could be used in the dry commercially available form.
In another aspect the present invention is directed to the manufacturing of artificially colored roofing granules coated on a substrate or carrier. The loadings in pounds per ton (ppt) base granules comprise:
Alternatively, if dry basis pigments are used, the loadings in pounds per ton (ppt) base granules comprise:
The preferred alkali silicate is sodium silicate of SiO2/Na2O weight ratio of 3.2 to maximize the extent of dehydration and insolubilization at the low temperature of about 400xc2x0 F. firing to produce as transparent coating as possible. This helps highlight the reflective/decorative characteristics of the metallic flake or lamellar effect and reduces tarnishing of the metal surface during the granule manufacturing process.
For achieving special color effects in roof granules, a two-coat product is made which comprises a high-temperature pigmented first coat and a second coat described above. For granules with the appearance of metallic aluminum, the first coat preferably is a white or light gray base which extends the bright silvery metallic appearance of the second coat.